An intense interest has been paid to a coating film in which a continuous phase is formed of an inorganic material such as a metal oxide and the like as a next generation coating material, because the coating film has high hardness and flame retardancy which cannot be achieved by a coating film in which a continuous phase is formed of an organic material. Further, in addition to these characteristics, such an inorganic coating film has excellent solvent resistance, light resistance, weather resistance, and the like, and can also be provided with functions such as superhydrophilicity, superhydrophobicity, antistaticity, and the like. Therefore, its application is greatly anticipated.
As the inorganic coating film, a coating film in which a continuous phase is formed of metal oxide formed by a sol-gel reaction has been widely studied. In particular, these metal oxide coating films having a regular structure inside and outside the coating film are mostly a organic/inorganic composite coating film, wherein an organic polymer is hybridized in a matrix of a metal oxide. These originate from the study of a biosilica. That is, in a recent study of the biosilica, it was found that the cell membrane of diatoms is basically composed of silica, and that the silica cell membrane has an extremely precise pattern from nanoscale to micron-scale. It has been also found that polyamines are highly associated with the derivation of the pattern (see M. Hildebrand, Progress in Organic Coatings, 2003, Vol. 47, p. 256-266). If such a precise pattern of the biosilica can be realized in the inorganic coating film, it becomes highly possible to construct the coating film with various devices such as a biosensor, an optical material, an electronic material, a functional catalyst material, and the like. Therefore, an inorganic material having a precise pattern or a spontaneous pattern formation without processing the inorganic material has been studied. Further, since if an organic/inorganic composite coating film which is robust and has a precise pattern is calcined at a high temperature, only the organic compound is removed while maintaining the precise pattern, and therefore, it is believed that if the pattern can be controlled, it is also possible to obtain a material having a structural color including an inorganic material.
For example, a silica block having numerous holes on the surface and a diameter of several hundred nanometers or more, which is prepared by using a biomolecule isolated from a biosilica has been disclosed (see N. Poulsen et al., Proc. Natl. Acad. Sic. USA, 2003, Vol. First 100, p. 12075-12080). The silica block has holes on the surface, but the holes have more or less different diameters, and thus the patterns are not controlled.
Furthermore, there has been reported that a composite film of silica and an organic polymer has been obtained by fixing a molecule having a polymerization initiation ability onto the surface of gold, polymerizing a polymerizable monomer having an amino group to form a number of amine polymers on the surface of the gold in the form of a brush, and then performing a hydrolysis/condensation reaction of an alkoxysilane on the amine polymer brush (see Don Jin Kim et al., Langmure, 2004, Vol. 20, p. 7904-7906). The surface of the composite coating film thus obtained did not have a flat structure, but had a fine peak-valley structure on a nanometer scale. However, the irregular peak-valley structure was formed from the aggregation of silica particles, and its surface shape was formed at random and therefore, a precise pattern was not formed thereon.
As a method for obtaining an organic/inorganic composite coating film having holes formed on the flat film surface, a method for applying a coating composition including an aqueous polymer having a polyamine segment and a metal alkoxide has been disclosed (see, for example, WO 2006/011512 (“WO '512”)). The pore diameter or depth of the holes of the coating film surface depends on the structure, molecular weight, or the like, of the aqueous polymer used as a material. Accordingly, in the case where the obtained coating film is used for fixing a functional compound, it is necessary to form a hole corresponding to the functional compound, and therefore, it is necessary to make various studies on the structure and the like of the aqueous polymer. Furthermore, in the case where a compound having a fluorescence or a coloring property is introduced to a water soluble polymer so as to provide the coating film with a new function, the shape of the formed hole changes, and therefore, it is insufficient for general applicability. Further, since the coating film does not have a hollow structure therein and the aqueous polymers are not regularly arranged, the color of the metal oxide only is developed even after calcination of the coating film, and accordingly, the structural color is not developed.
In addition, it has been disclosed that an organic/inorganic composite coating film having a uniform distribution of the core-shell particles in the coating film is obtained by applying a coating composition including a water dispersible core-shell particle having an amino group-containing polymer as a shell layer and a silane compound (see, for example, JP-A-2006-291089 (“JP '089”)). The coating film forms a dense hybrid structure having a polymer particle interface and a silica matrix. However, the holes are not formed on the film surface, nor do they have a hollow structure therein. Therefore, in the case of having the functional compounds in combination in order to provide various functions for the composite coating film thus obtained, the amino group on the core-shell particle surface is modified. As a result, the sol-gel reaction of the silane compound is affected, and correspondingly, the physical properties of the organic/inorganic composite coating film thus obtained are also affected, which limits the range of applications. Moreover, even though the coating film is burned to remove the core-shell particles, the particles are not regularly arranged, and therefore, the structural color is not developed.
On the other hand, a method for obtaining a coating film having a structural color, has been proposed, for example, a technology in which other materials are interpenetrated and charged in the void in a fine particle array film using the fine particle array film as a template (cast), and then the fine particles are removed to form a periodically porous structure, which is used as a structural color film. For example, a colloidal crystal is prepared by the suction filtration of a dispersion of fine particles of polystyrene, and a solution of a metal alkoxide is dropped thereon and penetrated between the fine particles. The method in which by calcination of this, a continuous form of a structure of metal oxide between fine particles is formed, and thereafter, the polystyrene is removed to prepare an inverse opal structure (for example, see Brian T. Holland et al., “Science”, Vol. 281, 1998, p. 538-540 (“Holland”)), or the like has been disclosed.
However, in the method of Holland, there has been a problem that after the preparation of the colloidal crystal, the metal alkoxide is packed in the densely packed and very narrow void of the particles, and thus, when the void portion of the surface is packed with these materials, nothing can penetrate thereinto, and therefore, the void between the particles is not sufficiently packed, thereby giving an inhomogeneous periodic structure. In addition, since the surplus metal alkoxide, which is not packed, forms a continuous body having no periodic structure by calcination, this case suffers a problem that a non-uniform material having a portion showing a periodic structure and a portion not showing a periodic structure in the mixture is provided. Further, since a part of a three dimensional periodic structure having an inverse opal structure uses a template having particles coming in contact with each other, a fragile structure having pores connected at the contact points is formed, thereby causing the generation of cracks from the shrinkage involved with calcination. Thus, it is difficult to maintain the structure. The problems with the inhomogeneity and the strength get more serious as the size of the material increases, and accordingly, it is basically difficult to prepare a structural color film with a large area.
In order to solve the problems with the inhomogeneity and the strength, the present inventors have already disclosed a technology involving adding a metal alkoxide to a sol, in which a core-shell particle having a fine particle as a core portion and a crosslinked hydrophilic organic polymeric compound as a shell portion is dispersed in water or a hydrophilic solvent, to conveniently prepare a three dimensional periodic structure having the organic material and the inorganic material combined therewith by a sol-gel reaction of the alkoxide, and burning the structure to remove the organic component, thereby obtaining a periodic structure having an inverse opal structure comprising a metal oxide (see JP-A-2006-213534 (“JP '534”)). Since this technology uses a crosslinked shell layer having a certain thickness which forms a hydrogel as a reaction field of a sol-gel reaction while not drying a film having the fine particles arranged therein, a layer of a metal oxide which is uniform between the particles and has a sufficient thickness can be easily formed. By this, there is no contact between adjacent pores and a rigid inverse opal structure can be obtained, and therefore, it becomes possible to obtain a three dimensional periodic porous structure which is relatively larger than ones formed using a conventional method.
In the method proposed in JP '534, a dispersion sol of the core-shell particles is applied on a substrate, and immersed in a metal alkoxide to proceed a sol-gel reaction in the shell layer, and fix the array of the core-shell particles. Accordingly, the usable substrate is limited to a material which is stable to the metal alkoxide. Moreover, since a step for dipping the entire substrate is involved, it is necessary to prepare a large dipping tank adapted for the size of the substrate in order to obtain a large-scale structural color film. Further, it is necessary to prepare a large amount of the metal alkoxide in order to fill the large-scale dipping tank. As seen from this, the preparation method proposed in JP '534 had a problem related to practical use to still be solved. Further, the three dimensional periodic structure including a core shell type fine particle prepared on a substrate by such a method forms a three dimensional periodic structure of a strong organic/inorganic composite by a sol gel reaction in the shell layer, but a strong adhesion with the substrate based on a covalent bond does not originally exist. Accordingly, when a large structural color film is prepared, there may be a case of peeling from the substrate with the progress of the sol-gel reaction or a case of peeling from the substrate upon calcination. In the case of peeling off the film as above, a local stress is applied, and as a result, the obtained structural color film is folded or wound, and thus cracks are easily generated in the film. Therefore, it is difficult to obtain a structural color film or a structural color coating on substrate.
As such, an organic/inorganic composite coating film having a robust structure, in which the internal porous structure and a surface pattern structure are highly controlled at the same time, has not been realized yet. Accordingly, a structural color film with a large area and having no defects, cracks, or the like, from which the organic/inorganic composite coating film will be obtained by calcination, has not been found.
It is a problem solved by the present invention to provide an organic/inorganic composite coating film comprising an organic material combined with a matrix comprising an inorganic material, wherein the coating film has a periodic porous structure therein, and the coating film surface has a semispherical peak-valley surface profile on the surface, a method for conveniently preparing the composite coating film, and a structural color film with a large area obtained by burning the composite coating film.